When designing and manufacturing microwave equipment including low noise amplifiers, the active element, such as a field effect (FET) transistor, must be properly connected to a circuit that presents the optimum source and load impedance to its input and output terminals. Some means must be provided to alter these impedances to compensate for variations in the FET's and in specific circuit dimensions. The higher the frequency involved in the microwave circuit, the more critical these variances become. For microwave low noise amplifier (LNA) circuits that use GaAs MESFET transistors on microstrip circuits, some means must be provided to trim the matching circuit geometry at the transistor terminals to compensate for changes in the optimum source and load impedances of the transistors from different lots. One method of accomplishing this tuning and transistor characteristic matching may be found in a patent granted to the inventor and assigned to the same assignee as the present invention and given U.S. Pat. No. 4,472,690.
Some microwave circuits using ceramic substrates are designed with the FET devices mounted to the ground plane using flanges that may be bolted to a metal plate. Other FET devices are physically mounted using solder type attaching mechanisms. However, in either case, the ceramic substrate that is connected to the FET must be soldered to a common ground plane to ensure intimate contact of the substrate ground plane with the FET ground flange to the chassis ground plane. The impedance matching stubs for a typical FET are etched or plated on a dielectric substrate causing them to present fixed impedances to the FET. When a FET is replaced by a different FET, the different noise parameters require that the impedance matching stubs be retuned using suitable metal or dielectric chips. The tuning operations can be expensive and require trained personnel to carefully assemble and test the amplifiers.
Although the above approaches have worked at frequencies of up to six GHz, the problems associated with the prior art approach are amplified even further as the frequency is extended beyond six GHz.
The gain of the active component such as the FET, which starts at a mediocre value in FET's of today's technology, is typically reduced even more by negative feedback and by the various components used to produce a viable circuit. Thus, to get the gain required for the overall circuit, it is often necessary to add additional active components such as additional FET's.
Additionally, it is often desirable to shield the field radiated by an active circuit working at high frequencies, such as 11 GHz, so that not only is it not affected by signals from other sources, but additionally so that signals radiated by the active components do not affect other nearby circuitry.
The present invention includes a metal shield which is placed over the FET and partially encloses the space above the ground plane, thereby not only shielding signal radiations both received and generated but, also providing positive space signal feedback wherein the gain of the circuit is increased over that normally expected while simultaneously reducing the noise figure of the circuit.
Prior to the present invention, it has been generally accepted by microwave engineers that positive feedback will raise the noise figure of an amplifier even though it will increase gain. This increase in gain is typically at the expense of stability. In other words, the higher the feedback, the poorer the stability. Articles such as that found in "Microwave", October 1978, beginning on page 66, typically do not discuss the noise figure in great detail, since they do not appreciably improve the noise figure.
The present invention not only increases the gain through the use of the positive space feedback, while simultaneously providing a shielding effect but, in addition, reduces the noise figure.
It is thus an object of the present invention to improve upon the operation of a high frequency RF amplifier.